What is "temperature compensation"?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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How can we remove annoying temperature interference through algorithms or structures?

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In Fiber Bragg Grating (FBG) sensing technology, the reflection center wavelength of an FBG responds to both temperature and strain simultaneously due to the thermo-optic and thermal expansion effects of the optical fiber’s silica material. This phenomenon is known as Temperature Cross-sensitivity.

To eliminate this troublesome temperature interference, the field of optical engineering typically addresses it from two main dimensions: Algorithms (Active/External Temperature Compensation) and Structures (Passive Mechanical Compensation or Integrated Dual Gratings).

I. Algorithmic and External Sensor Compensation Method (High Precision, Engineering Preferred)

This is currently the most common and accurate solution in industrial and research fields. The basic idea is to introduce an FBG temperature sensor that is only affected by temperature and not by any external mechanical forces next to the strain (force/pressure) measurement point.

1. Compensating Algorithm Mathematical Model

Let the wavelength shift of the strain sensor (subject to both mechanical strain and temperature changes) be \Delta \lambda_{\text{strain}} , and the wavelength shift of the external temperature sensor (only experiencing temperature changes) be \Delta \lambda_{\text{temp}} :

\Delta \lambda_{\text{strain}} = K_{\epsilon} \cdot \epsilon + K_{T,\text{strain}} \cdot \Delta T
\Delta \lambda_{\text{temp}} = K_{T,\text{temp}} \cdot \Delta T

Where K_{\epsilon} is the strain sensitivity coefficient, and K_{T,\text{strain}} and K_{T,\text{temp}} are the temperature sensitivity coefficients of the strain sensor and temperature sensor, respectively. By solving the equations to eliminate the temperature term \Delta T , the true physical strain value \epsilon , completely free from temperature interference, can be demodulated:

\epsilon = \frac{1}{K_{\epsilon}} \left( \Delta \lambda_{\text{strain}} - \frac{K_{T,\text{strain}}}{K_{T,\text{temp}}} \cdot \Delta \lambda_{\text{temp}} \right)

Since the temperature sensors from Dacheng Yongsheng are calibrated with high-precision polynomial fitting for temperature-wavelength shifts at the factory, you can also directly substitute the real temperature value T calculated by the demodulator into the strain sensor’s temperature correction equation in real-time.

2. Corresponding Hardware Deployment

To implement this high-precision algorithmic compensation, the following highly reliable FBG sensors are recommended for use at the measurement point:

II. Structural Self-Compensation Method (Passive Adaptive)

If you prefer to avoid complex data fusion between channels at the demodulator or in the algorithm software, temperature drift can also be canceled out at the hardware level through the sensor’s internal physical structure design.

1. Differential Thermal Expansion Coefficient Compensation Structure (Passive Mechanical Compensation)

This method utilizes the difference in coefficients of thermal expansion ( \alpha ) between different solid materials.

  • Structural Design: The fiber grating is suspended and fixed inside a sleeve structure composed of two types of metals with different coefficients of thermal expansion (e.g., an aluminum alloy tube with a high expansion coefficient and an Invar alloy/special steel tube with a very low expansion coefficient).
  • Cancellation Mechanism: When the ambient temperature rises, the thermo-optic effect of the fiber originally causes the reflection wavelength to drift towards the infrared (redshift); however, the composite sleeve with a significant difference in thermal expansion coefficients will exert a slight